Re: [PATCH V4 00/27] ublk: add UBLK_F_BATCH_IO

Caleb Sander Mateos
2025-11-28T19:07:30Z

On Fri, Nov 28, 2025 at 8:19 AM Jens Axboe <axboe@kernel.dk> wrote:
>
> On 11/28/25 4:59 AM, Ming Lei wrote:
> > On Fri, Nov 21, 2025 at 09:58:22AM +0800, Ming Lei wrote:
> >> Hello,
> >>
> >> This patchset adds UBLK_F_BATCH_IO feature for communicating between kernel and ublk
> >> server in batching way:
> >>
> >> - Per-queue vs Per-I/O: Commands operate on queues rather than individual I/Os
> >>
> >> - Batch processing: Multiple I/Os are handled in single operation
> >>
> >> - Multishot commands: Use io_uring multishot for reducing submission overhead
> >>
> >> - Flexible task assignment: Any task can handle any I/O (no per-I/O daemons)
> >>
> >> - Better load balancing: Tasks can adjust their workload dynamically
> >>
> >> - help for future optimizations:
> >> - blk-mq batch tags free
> >> - support io-poll
> >> - per-task batch for avoiding per-io lock
> >> - fetch command priority
> >>
> >> - simplify command cancel process with per-queue lock
> >>
> >> selftest are provided.
> >>
> >>
> >> Performance test result(IOPS) on V3:
> >>
> >> - page copy
> >>
> >> tools/testing/selftests/ublk//kublk add -t null -q 16 [-b]
> >>
> >> - zero copy(--auto_zc)
> >> tools/testing/selftests/ublk//kublk add -t null -q 16 --auto_zc [-b]
> >>
> >> - IO test
> >> taskset -c 0-31 fio/t/io_uring -p0 -n $JOBS -r 30 /dev/ublkb0
> >>
> >> 1) 16 jobs IO
> >> - page copy: 37.77M vs. 42.40M(BATCH_IO), +12%
> >> - zero copy(--auto_zc): 42.83M vs. 44.43M(BATCH_IO), +3.7%
> >>
> >>
> >> 2) single job IO
> >> - page copy: 2.54M vs. 2.6M(BATCH_IO), +2.3%
> >> - zero copy(--auto_zc): 3.13M vs. 3.35M(BATCH_IO), +7%
> >>
> >>
> >> V4:
> >> - fix handling in case of running out of mshot buffer, request has to
> >> be un-prepared for zero copy
> >> - don't expose unused tag to userspace
> >> - replace fixed buffer with plain user buffer for
> >> UBLK_U_IO_PREP_IO_CMDS and UBLK_U_IO_COMMIT_IO_CMDS
> >> - replace iov iterator with plain copy_from_user() for
> >> ublk_walk_cmd_buf(), code is simplified with performance improvement
> >> - don't touch sqe->len for UBLK_U_IO_PREP_IO_CMDS and
> >> UBLK_U_IO_COMMIT_IO_CMDS(Caleb Sander Mateos)
> >> - use READ_ONCE() for access sqe->addr (Caleb Sander Mateos)
> >> - all kinds of patch style fix(Caleb Sander Mateos)
> >> - inline __kfifo_alloc() (Caleb Sander Mateos)
> >
> > Hi Caleb Sander Mateos and Jens,
> >
> > Caleb have reviewed patch 1 ~ patch 8, and driver patch 9 ~ patch 18 are not
> > reviewed yet.
> >
> > I'd want to hear your idea for how to move on. So far, looks there are
> > several ways:
> >
> > 1) merge patch 1 ~ patch 6 to v6.19 first, which can be prep patches for BATCH_IO
> >
> > 2) delay the whole patchset to v6.20 cycle
> >
> > 3) merge the whole patchset to v6.19
> >
> > I am fine with either one, which one do you prefer to?
> >
> > BTW, V4 pass all builtin function and stress tests, and there is just one small bug
> > fix not posted yet, which can be a follow-up. The new feature takes standalone
> > code path, so regression risk is pretty small.
>
> I'm fine taking the whole thing for 6.19. Caleb let me know if you
> disagree. I'll queue 1..6 for now, then can follow up later today with
> the rest as needed.

Sorry I haven't gotten around to reviewing the rest of the series yet.
I will try to take a look at them all this weekend. I'm not sure the
batching feature would make sense for our ublk application use case,
but I have no objection to it as long as it doesn't regress the
non-batched ublk behavior/performance.
No problem with queueing up patches 1-6 now (though patch 1 may need
an ack from a kfifo maintainer?).

Thanks,
Caleb
>
> --
> Jens Axboe